Implantable medical device (IMD) systems used for monitoring cardiac signals or delivering electrical stimulation therapy often employ electrodes implanted in contact with the heart tissue. Such electrodes may be carried by transvenous leads to facilitate implantation at endocardial sites or along a cardiac vein. Epicardial leads, on the other hand, carry electrodes adapted for implantation at an epicardial site. In past practice, placement of transvenous leads is often preferred by a physician over epicardial lead placement since transvenous leads can be advanced along a venous path in a minimally invasive procedure. Epicardial lead placement has generally required a sternotomy in order to expose a portion of the heart to allow implantation of the epicardial electrode at a desired site.
However, depending on the particular application, an epicardial lead may provide better therapeutic results than a transvenous lead. For example, in cardiac resynchronization therapy (CRT), a transvenous lead is advanced through the coronary sinus into a cardiac vein over the left ventricle. Implantation of a transvenous lead in a cardiac vein site can be a time-consuming task and requires considerable skill by the implanting clinician due to the small size and tortuosity of the cardiac veins. Furthermore, implant sites over the left heart chambers are limited to the pathways of the accessible cardiac veins when using a transvenous lead, which does not necessarily correspond to therapeutically optimal stimulation sites. Epicardial electrodes are not restricted to the pathways of the cardiac veins and can be implanted over any part of the heart surface. In order to take full advantage of cardiac stimulation therapies such as CRT, it is desirable to offer a cardiac lead that can be implanted in an epicardial location and a delivery system that allows the lead to be implanted using a generally less invasive approach, such as a mini-thoracotomy, a sub-xyphoid, or thorascopic approach, than a full sternotomy.
Helical fixation members for screwing into myocardial tissue are employed in epicardial pacing and defibrillation leads and the construction of such medical electrical leads are well known in the art. Typically the helical member terminates a distal end of the lead and serves as a stimulating electrode. A need remains for improvements to the delivery mechanisms to facilitate delivery of the distal member of the lead, such as a helix, for fixation of the lead to the epicardial surface of the heart.